Condenser bushing



J. H. FRAKES CONDENSER BUSHING Filed Feb. 20, 1940 2 Sheets-Sheet 1\vakk A QQ N 4 3 Q2 Q3 9:

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WlTNESSES: 697% Patented Nov. 25, 1941 CONDENSER BUSHING James H.Frakes, Edgewood, Pa., assignor to Westinghouse Electric 86Manufacturing Company, East Pittsburgh, Pa., a corporation ofPennsylvania Application February 20, 1940, Serial No. 319,869

2 Claims.

My invention relates to high voltage insulating bushings and it hasparticular relationship to oil filled bushings of the condenser type.

In a conventional type of bushing a conducting stud extends through aporcelain casing and a number of porcelain insulating tubes are arrangedabout the stud within the casing. Even though filled with oil such abushing must be rather thick to provide the required insulatingproperties and, in addition, the voltage gradient necessarily variesconsiderably throughout. Because of the difllculty in forming porcelainaccurately to a predetermined size, it is difilcult to obtain a uniformand accurate fit between the various insulating tubes.

In accordance with invention, I obtain a condenser effect by providingcoatings of conducting material on portions of the inside and outsidesurfaces of the insulating tubes in a bushing as above described, andpreferably the adjacent cOnducting coatings of adjoining tubes areelectrically connected together to avoid high stresses on the materialin the spaces therebetween. In addition I make the alternate insulatingtubes of ceramic and fibrous materials which, because fibrous materialmay be readily machined, makes possible a more accurate fit. As a resultof the condenser effect and a more satisfactory fit between theinsulating tubes, it is possible to reduce the thickness of a bushingrequired to furnish insulation at a particular potential.

It is accordingly an object of my invention to provide a novelinsulating bushing supplying increased insulating properties for eachunit of space required thereby.

It is another object of my invention to provide an insulating bushing inwhich the insulating medium includes alternate fibrous and ceramictubular members.

A further object of my invention is to provide a condenser typeinsulating bushing in which a conducting material is secured to thesurfaces of tubular dielectric members.

Other objects and advantages of my invention will appear from thefollowing description taken in conjunction with the accompanvingdrawings in which:

Figure 1 is a view in longitudinal section of an insulating bushingbuilt in accordance with my invention;

Fig. 2 is a transverse sectional view of the bushing shown in Fig. 1;

Fig. 3 is a diagram showing the voltage gradient and distributionthroughout the bushing shown in Figs. 1 and 2;

Fig. 4 is a diagram similar to Fig. 3 for a bushing having the samedimensions as that of Figs. 1 and 2, but in which no condenser effect isprovided, and

Fig. 5 is a diagram similar to Figs. 3 and 4 of a non-condenser typebushing in which liquid filled spaces of considerable thickness existbetween adjacent insulating tubes.

In Figs. 1 and 2 is shown an insulating bushing having an outer casingmade up of an upper member I and a lower member 2, both of which may bemade of insulating material such as porcelain. A conducting stud 3.which ma; be a copper tube or rod, extends axially through the outsidecasing and has a cap 4 sealed to it near its upper end. The cap 4 isdisposed on the open upper end of the upper casing member I with asealing gasket 5 therebetween. This cap is preferably provided with aplug 6 to facilitate filling the bushing with an insulating liquid. Anut I threaded 0n the lower end of the stud 3 secures the lower casingmember in assembled relationship with a mounting fiange or supportingmember 8 therebetween. Gaskets are provided at 9 and III to formliquid-tight seals between the two casing members and the flange 8 andbetween the lower casing member 2 and the stud 3, respectively.

Within the outer casing a fiber insulating sleeve or tube Ii fits aroundthe stud 3. Porcelain insulating tubes I2 and I3 are concentricallyarranged around the stud 3 with another fiber tube It disposed betweenthem. While these insulating tubes are shown supported at their lowerends by the lower casing member 2, another suitable supporting means maybe employed if desired. The outermost insulating tube I3 is preferablybrazed to the supporting flange 8 and held in position in this manner.

A coating 15 of conducting material is disposed on the exterior of thefiber sleeve II for a portion of its length. Similar coats of conductingmaterial I6 are secured to the inner and outer surfaces of the otherinsulating tubes l2, l3 and it. These conducting coats may be depositedin any 01' a number of different ways as, for example, by the well knownmetal spray process. The coating IS on the innermost insulating tube IIis electrically connected by means of slugs of solder I1 to theconducting coating [6 on the inner surface of the porcelain insulatingtube l2. In like manner the coating on the exterior surface of the tubeI2 is electrically connected to the coating on the interior of the tubeit, and the exterior coating on the tube It is connected to the interiorcoating on the tube I3. If preferred, other means such as metallic ringsor springs disposed between the adjacent tubes may be used instead ofthe solder H to form the desired connections.

The length of the various conducting coverings l5 and I6 may be variedor graded as shown in the drawings to control the voltage gradientthroughout the bushing. The coatings which are electrically connected toeach other are preferably formed with substantially equal area sincethey are maintained at equal potentials.

It will be seen that the metallic coatings and the electricalconnections result in a condenser arrangement throughout the bushing inwhich the insulating tubes form the dielectric between plates. Thebushing is preferably filled with an insulating liquid, the primarypurpose of which is to prevent creepage along the uncoated ends of thevarious insulating tubes. The insulating liquid will also aid in coolingthe apparatus.

By comparing the graphs shown in Figs. 3 and 4, the advantages of thecondenser arrangement in the bushing above described may be observed.Fig. 3 shows the voltage gradient and voltage distribution at aflashover value of 265 kilovolts for a bushing such as shown in Figs. 1and 2. Fig. 4 shows th same quantities under the same conditions for abushing of the same size in which no condenser effect is provided. Thesuperiority of the described bushing is indicated by the fact that themaximum gradient to which its insulation is exposed amounts to 285 voltsper mi1l,' while the bushing represented in Fig. 4 is exposed to agradient of 380 volts per mill. In addition, the insulation of the firstbushing is more effectively utilized since the gradient to groundamounts to 120 volts as compared to 94 in the second device. When thecondenser effect is employed the same size bushing will furnishinsulation for much higher voltages or a bushing of smaller dimensionsmay be utilized to provide the same protection.

Most insulating liquids have a lower specific inductive capacity thansolid insulating materials. When an insulating structure is made up ofsolid and liquid materials in series the liquid will take the greaterpart of the stress because the voltage is distributed in inverseproportion to th specific inductive capacities of these materials. As aresult of the relatively low dielectric strength of the liquid, it ispreferred'not to use it directly as a dielectric medium between theconducting coatings which in practical effect are condenser plates. Bymeans of the electrical connections between the adjacent conductingsurfaces, such use is avoided. The effect which the omission of theseconnections would have on the bushing may be observed from Fig. 5 whichshows the voltage gradient and voltage distribution throughout a bushingof slightly different dimensions from those previously described and inwhich an oil gap of considerable thickness exists in series with theother insulation. As shown at A and B on this figure, the oil in thesegaps is stressed to a greater extent than the adjoining solidinsulation.

In addition to the functional superiority above described, my inventionoffers advantages in its construction and assembly. Because of thereadiness with which the fiber sleeves II and I4 between the porcelainsleeves may be machined, it is a relatively simple matter to secure aclose and uniform fit. The ease with which the bushing may be assembledis believed to be evident from the above description.

While I have shown and described a particular embodiment of myinvention, it will be evident that modifications may be made thereinwithout departing from its spirit and scope as set forthin the followingclaims.

I claim as my invention:

1. An insulating bushing comprising a tubular casing of insulatingmaterial, an electrical conductor extending through said casing, and aplurality of tubular members of dielectric material disposed within oneanother about said conductor, the innermost of said tubular membersbeing of fibrous material and disposed in snug engagement with saidconductor while the other of said tubular members are alternately ofceramic and fibrous material.

2. An insulating bushing comprising a tubular casing of insulatingmaterial, an electrical conducting stud extending axially through saidcasing, a plurality of tubular members of dielectric material disposedconcentrically about said conducting stud within one another, theinnermost of said tubular members being of fibrous material and inengagement with said conducting stud while the other of said tubularmembers are alternately of ceramic and fibrous material, electricalconducting material on the outside surface of said innermost tubularmember and on the inside and outside surfaces of said other tubularmembers, means electrically interconnecting the conducting material onthe adjacent surfaces of adjacent tubular members, and an insulatingfiuid disposed in said casing about said tubular members.

JAMES H. FRAKES.

